The mouse Twirler mutation on chromosome 18 causes circling behavior due to abnormal inner ear development in Tw/+ heterozygotes, and cleft lip and palate in Tw/Tw homozgyotes. We are genetically mapping Tw on chromosome 18 using a large mapping backcross. We plan to attempt to identify the Tw gene by positional cloning or positional candidate gene approaches. Our current mapping data places Twirler approximately 1 centimorgan from the centromere of mouse chromosome 18.